How to test O2 Sensor?
 

Is there a way to test to make sure my O2 sensor is working properly... or given that I have no history on my recently purchased 89 metro, should I just assume it needs to be replaced?

Well the 2 ways I know of on toyotas:
With the car warmed up, rev the engine and measure the voltage between the O2 sensor output and ground. It should oscillate several times a second as the engine overshoots and undershoots the "correct" fuel-air ratio.

The other way is to take it out, and measure the voltage on the output wire while heating just the sensing end of the sensor with a propane torch. You should see a voltage output, because, as far as I understand it, the O2 sensor is a battery which is powered by O2 concentration *differences* between the sensing end, and the "outside the exhaustpipe" end. Which is why those little holes are drilled in the side of the outside end: gives access to the atmospheric O2 concentration.

There is a O2 sensor problem that I don't think either of these tests will find, and that is when the O2 sensor develops a voltage offset which skews all the readings high or low.

If there's a problem, it will be obvious. A CEL will go off, it will run like garbage, and/or stink of gas.

When measuring the voltage use a high impedance volt meter(20megohms).
The voltage signal is usually between 0.2 and 0.8 volt DC. 14.7 to 1 air/fuel is somewhere between 0.3 and 0.6 VDC. So the sensor is so touchy in the midrange that all it can do is switch between rich and lean. <0.3 lean and >0.6 rich.

The two white wires are the heater prw and grnd.(Ford)
The gray is signal and the black is signal grnd on a 4 wire Ford.(Bosch)

Johnny do you have the wire colors on other brands? Also is it correct to say that a failing sensor would put out less voltage?